Bagasse, a fibrous residue from sugar cane processing, has traditionally been used as a fuel to provide motive auxiliary power for sugar mill operations. This has been accomplished by burning the bagasse in a boiler and using the steam as a direct power source and/or converting the steam into electrical energy for in house use and, when appropriate, export. Boiler efficiencies are low with most losses occurring in stack flue gas.
During recent times, as fossil fuels have become scarcer and more costly, efforts have been made to increase the efficiency of bagasse conversion into both kinetic and potential energy forms. Generally these efforts have been in the area of drying the bagasse through various methods thereby increasing its combustion efficiency or increasing the efficiency of the boilers by using higher steam pressures, better use of insulation, optimization of combustion techniques, etc. Utilities must accept electrical energy from non-fossil sources when that energy is provided continuously year round and at a specified minimum rate. Electrical generation in sugar factories in the past have provided only short term variable electrical power to the utility company, which reduced the desirability of this energy to the utility.
The drying and compacting of cellulosic wastes to improve their utility for fuel and other purposes is known in the art. Patents in this field include British Pat. No. 20,312 of 1911, British Pat. No. 14,983 of 1915, U.S. Pat. No. 1,258,888 to Geveke, U.S. Pat. Nos. 2,786,760 and 2,870,481 to Bonnafoux, U.S. Pat. No. 2,995,445 to Briggs et al, U.S. Pat. No. 3,013,880 to King, U.S. Pat. No. 3,586,511 to Bishop, U.S. Pat. No. 3,946,660 to Kuhtreiber, U.S. Pat. No. 4,015,951 to Gunnerman and U.S. Pat. No. 4,236,897 to Johnston. Of these patents, the British Pat. No. 14,983 and U.S. Pat. No. 1,258,888 to Geveke specifically relate to the processing of bagasse or, in the case of the British patent, megass. These patents indicate that there are serious problems in handling this material. The Johnston U.S. Pat. No. 4,236,897 mentions bagasse among a number of other natural cellulosic materials which may be pelleted by combining them with thermoplastic polymeric material.
The British Pat. No. 14,983 discloses that it has been found impossible with mill megass to produce a satisfactory briquette without reducing the water content artificially to about 5 or 6%. To avoid this, the patent proposes to cut the megass into lengths of about 1/4 of an inch, squeeze out 60 to 80% of the sweet liquid, steam the resulting squeezed material and then briquette it under heavy pressure.
The patent to Geveke discloses the baling of wet bagasse from the crusher mills, permitting the bales to dry and then cutting the bagasse into small pieces and briquetting it. The amount of moisture is not specified.
The patent to Bishop deals with sugar cane and relates to the making of an animal feed rather than a fuel pellet. In the proposed process sugar cane is burned in the field by controlled burning to burn off dead, dry leaves and simultaneously decrease the moisture content of the remaining stalks. The stalks are then harvested, chopped into particles sizes ranging from fine to approximately 1/4 of an inch, dried to 1 to 3% moisture, rehydrated to about 3 to 8% moisture and pelleted by extrusion through a 1/4 inch die.
These early patents relating to bagasse and sugar stalks indicate that there is a special problem involved in briquetting this type of material and that special procedures are necessary, including steam digestion as in the British Pat. No. 14,983 or drying and rehydrating as in Bishop U.S. Pat. No. 3,586,511. These patents would also indicate that it is necessary to reduce the moisture content prior to briquetting to about 5 to 6% (British Pat. No. 14,983) or 3 to 8% (the Bishop patent).
The patent to Johnston U.S. Pat. No. 4,236,897, which includes bagasse as one of the materials which can be pelleted by the process disclosed in that patent, requires that from about 1 to 50% by weight of polymeric thermoplastic material be present as well as the cellulosic waste material. The cellulosic material must be comminuted to a particle size such that all of the particles are minus 5 mesh and at least 50% of the particles are minus 10 mesh. There are no specific examples of treating bagasse.
The patent to Bonnafoux U.S. Pat. No. 2,786,760 discloses that alfalfa hay may be ground in a hammer mill to relatively small fragments, the moisture content adjusted as by means of steam, and the resulting material subjected to extrusion during which the temperature may rise to 125.degree. to 210.degree. F. The exact amount of moisture is not specified.
The Briggs et al U.S. Pat. No. 2,995,445 discloses a process for making a wafer or briquette from alfalfa hay and recommends a moisture content within the range of 12 to 25% with the preferred being approximately 15%. A product having a specific gravity within the range of 0.9 to 1.1 is produced.
The King et al patent discloses that conventional hay pelleting processes require the use of relatively dry hay having a moisture content of less than 14%.
The Gunnerman patent is primarily concerned with pelleting wood wastes and specifies comminuting the waste material to a particle size no more than 85% of the minimum dimension of the pellet to be produced and adjustment to 16 to 28% moisture before pelleting. It has been found that these conditions are not satisfactory for pelleting bagasse. The Gunnerman patented process is known in the trade as the "Woodex" process. A proposal for converting the "Woodex" process to process bagasse has been published in Sugar y Azucar, August 1980 issue and for purposes of background information the disclosure of this publication is incorporated herein.